How does radar work?

What is radar and how is it used to track aircraft?

Air traffic controllers mostly use secondary radar to track commercial aircraft and only use real radar in the case where transponders are not fitted, are turned off or are broken (Source: LuisALouro/iStockPhoto)

The word RADAR is an acronym for RAdio Detection And Ranging, and in its simplest form it consists of a transmitted radio signal aimed by an antenna in a particular direction, and a receiver that detects the echoes off any objects in the path of the signal, he says.

The transmitter consists of an electronic circuit that osciallates at a specific frequency, usually much higher than those frequencies used for radio or TV broadcasts, says Brooker.

This signal is sent out in short bursts of electromagnetic energy, called pulses, through the antenna which produces a narrow beam like that of a torch.

"Radar makes it possible to determine the direction to an object, generally referred to as the target, based on the direction the antenna is facing," says Brooker.

The distance to the target is determined from the time taken between transmitting the pulse and receiving the echo. This can be accurately determined because the radar signal travels at the speed of light, which is constant.

Air traffic control

For air traffic control radars, the beam is shaped like a fan, narrow in the horizontal direction, and wide in the vertical direction, to accommodate high-flying planes.

This beam scans around in a circle once every two or three seconds and echoes are displayed on a circular display called a plan-position indicator.

The air traffic controller — or a computer — can track the echoes or 'blips' on the display to determine where the aircraft is heading. This is called primary radar.

"Primary radar is seldom used any more in isolation as there are too many planes in the sky," says Brooker.

"These days, secondary radar is also used, in which a coded pulse sequence is sent to the aircraft and a transponder on the plane generates a coded return, containing a lot of information about the aircraft. This used to be called identification friend or foe, or IFF."

Air traffic controllers mostly use secondary radar to track commercial aircraft and only use real radar in the case where transponders are not fitted, are turned off or are broken.

"There was a case couple of decades back where a young man flew a light plane half way across the US without being detected as the air traffic controllers either had their primary radars turned off or thought his echo was from a flock of birds," Brooker says.

If the aircraft transponder is switched off, it can be difficult to identify which one of the many primary radar "blips" on the air traffic control display corresponds to the aircraft you are interested in, says Brooker.

"This may be why the transponder on flight 370 was apparently turned off at the range where the handover occurred from one air traffic control to another.

Limits to radar

Most people will have heard the expression 'flying below the radar'. This is named after a true phenomenon, Dr Brooker explains.

"It is caused by the interaction of the radar beam with the ground, which results in the beam 'lifting' off the horizon. If an aircraft is flying low enough, the beam hardly illuminates it and the range at which it can be seen is limited."

There are also limits to the distance over which radar can be used. The main problem with radar for long distance operation is the fact that the amount of power required to send and receive the signal is dependent on the distance to the aircraft raised to the power of four, says Brooker.

"Therefore if you want to double the range at which you can detect an aircraft, the amount of transmitted power must increase by a factor of 16."

Typical radars used to track planes out to a range of 100 kilometres or more transmit peak powers in the megawatts. However, the transmitted pulse is short, typically one micro second or so, and they only occur a few hundred times per second, so the average power is quite low.

For really long-range operation, the peak power required to send out the radar pulses become prohibitively large.

This has resulted in the development of innovations such as phased arrays that consist of a large number of smaller transmitters and receivers on a planar surface that operate in unison and pulse compression, which allows longer and lower power encoded pulses to be generated while still maintaining good range accuracy.

Another limitation to long-range radar is caused by attenuation through the atmosphere — even in clear air, but worse in the rain. This is inversely related to the wavelength of the signal, so long range radars operate at low frequency.

Hiding from radar

Electromagnetic waves "bounce" off objects that conduct electricity, so old-fashioned aircraft made from wood and canvas didn't produce big radar echoes, says Brooker. The same applies to modern planes made from carbon fibre composites. Aluminium skinned planes make the best targets.

"The shape of the aircraft is also important, and metal aircraft made from flat plates, sharp corners and edges generally produce strong echoes, so if you want to make an aircraft invisible, you can either make it from flat plates or facets that are aligned in such a way that the radar signals reflect away from the receiver. The F-117 stealth attack aircraft is an example of this."

Alternatively aircraft can be made without any right angles so that wings are blended into the body and external features are eliminated. Making an aircraft skin that absorbs radar energy using "radar absorbent materials" is another method to minimise the echo size, he says.

"The B-2 stealth bomber is probably the state-of-the-art, which uses most of these techniques, and provides an echo about as big as that produced by a bumble bee."

Dr Graham Brooker a radar engineer at the University of Sydney's School of Aerospace, Mechanical and Mechatronic Engineering. He was interviewed by Stephen Pincock.